1. Field of the Invention
The present invention relates generally to a concrete mixing plant and components thereof, and, particularly, without limitation, to such a plant that is portable for jobsite use.
2. Description of the Related Art
Pourable wet concrete material is readily available from numerous concrete plants that place various ingredients for mixing concrete material into a special purpose truck that mixes the ingredients into the concrete material as the truck is driven to the jobsite where the concrete material is needed. The concrete material must be finally placed within a matter of minutes after water is added to the ingredients and the mixing process is initiated, e.g., twenty minutes. Thus, the construction project must be situated within a twenty-minute drive of the concrete plant that places the ingredients into the trucks. Otherwise, the design strength characteristics of the concrete material, after hardening, may have been compromised. Projects which are located beyond the allowable haul time generally must seek a suitable concrete plant that can be erected at jobsite, thereby eliminating much of the transportation costs as the ingredients can then be hauled to jobsite in bulk.
One of the problems with erecting a concrete mixing plant at jobsite is the expense involved; only projects requiring substantial quantities of concrete can justify such an arrangement. Further, the various components of such plants are large and bulky, and generally require erection cranes for erecting those components, which adds to the overall concrete expense. Of course, the erection cranes must return to the jobsite when the components are being dismantled before being moved to another site.
Another problem with available portable jobsite concrete mixing plants is the procedure which must be followed in order to reduce airborne particulate contamination to acceptable levels. For example, cement and fly ash ingredients used for making concrete material consists of very fine particles. As a result, such particles easily become airborne, contaminating the surrounding atmosphere and surfaces.
Other problems with available concrete mixing equipment include undesirable accumulation of hardened concrete on mixing surfaces, segregation of ingredients due to dumping ingredients together instead of blending them together, and less than maximum space utilization due to inability to operate a mixer about a horizontal axis.
Although mixer trucks designed to haul the concrete material are available in various sizes, the maximum size permissible is limited by several considerations, such as physical size, maximum highway and bridge load limits, etc. As a result, substantial transportation costs must be absorbed in the price of the concrete. Further, numerous trucks must be closely coordinated to provide the substantial quantities of concrete material sometimes needed for larger construction projects.
During the early part of this century, concrete mixers were generally horizontally oriented as such a configuration permitted more efficient utilization of available space with width and height requirements remaining within acceptable limits. Unfortunately, such horizontally oriented mixers were difficult to keep clean and experienced substantial and unacceptable build-up of concrete material on the mixing surfaces. As a result, concrete mixer design trended toward a configuration where mixing occurred horizontally but the axis was tilted to effect discharge.
What is needed is a portable concrete mixing plant that eliminates contamination caused by generation of airborne particulate matter by minimizing those activities which are most conducive to such generation; a portable concrete mixing plant using weigh surge techniques whereby cycle time for mixing a batch of concrete material is substantially reduced; a portable concrete plant that can be easily and quickly moved to a jobsite and erected and dismantled without the use of erecting cranes; a portable concrete plant that utilizes a water dispensing system whereby water for mixing concrete material can be quickly transferred to a mixer while simultaneously cleaning interior surfaces of the mixer as an integral part of the mixing process; a concrete plant that does not dump ingredients together but, instead, blends the ingredients together; a concrete plant that gravimetrically measures ingredients for concrete material; and a mixer having a horizontal axis that can be efficiently operated without material building up on internal surfaces thereof.